In Canadian law, patently unreasonable or the patent unreasonableness test was a standard of review used by a court when performing judicial review of administrative decisions. It was the highest of three standards of review: correctness, unreasonableness, and patent unreasonableness. Although the term "patent unreasonableness" lacked a precise definition in the common law, it was somewhere above unreasonableness, and consequently it was relatively difficult to show that a decision is patently unreasonable. A simple example of a patently unreasonable decision may be one that does not accord at all with the facts or law before it, or one that completely misstates a legal test.
By a decision issued on March 7, 2008, this test was removed from the law by the Supreme Court of Canada in David Dunsmuir v. Her Majesty the Queen in Right of the Province of New Brunswick as represented by Board of Management .
In Toronto (City) Board of Education v. O.S.S.T.F., District 15,  1 S.C.R. 487, at paras. 41-48, the majority of the Supreme Court of Canada noted that the test for whether a decision under review is patently unreasonable is articulated differently for findings of fact and findings of law. For interpreting a legislative provision, the test was whether the decision under review "cannot be rationally supported by the relevant legislation and demands intervention by the court upon review". In the context of a decision interpreting a collective labour agreement, the patently-unreasonable test was held to mean that the court will not intervene unless the words of the collective agreement have been given an interpretation they cannot reasonably bear.
When the reviewing court reviews the evidence that was before the original decision maker, on a question where the standard of review is patent unreasonableness, the reviewing court must determine whether "the evidence reasonably viewed is incapable of supporting the tribunal's findings" (para. 48).
- Due process, the analogous standard of review in United States administrative law
- Wednesbury unreasonableness, the analogous standard of review in English administrative law
- Wednesbury unreasonableness in Singapore administrative law
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